The FBI has said it will go to the "ends of the Earth" to find those who used pressure cooker bombs to kill three and injure more than 170 people at the finish line of the Boston Marathon.

The pressure cooker bombs were hidden in duffel bags packed with nails, shards of metal and ball bearings and placed on the ground, around 100m apart along the finishing stretch of the Boston Marathon route, a source close to the investigation has disclosed.

They said that law enforcement officials had some of the bomb components but did not yet know what had been used to set off the explosives.

A 2010 Department of Homeland Security statement warns that terrorists could use pressure cookers to make bombs, saying that at least one of the three devices used in the May 2010 Times Square attack comprised a pressure cooker.

The second of the three victims killed in the attack has been named as 29-year-old restaurant manager Krystle Campbell, of Medford, who had been in Boston to take a picture of her boyfriend crossing the finish line.

Her father, William Campbell, says his daughter was "very caring, very loving person, and was daddy's little girl." He says the loss has devastated the family.

Video:Photos Capture Horror Of Explosions

President Barack Obama, who has announced he will visit the city on Thursday, labelled the twin-bombing an "act of terror".

Speaking at a news conference, he said: "What we don't yet know, however, is who carried out this attack or why, whether it was planned and executed by a terrorist organisation, foreign or domestic, or was the act of a malevolent individual."

He added: "What I have indicated to you is what we now know. We know it was bombs that were set off. We know that obviously they did some severe damage. We do not know who did them.

"We don't have a sense of motive yet. So everything else at this point is speculation."

Video:Obama: Bombings 'A Cowardly Act'

The FBI said it was following a number of leads in the wake of the attacks but special agent Richard DesLauriers declined to say whether anyone was in custody.

Mr DesLauriers said: "We will go to the ends of the Earth to identify the subject or subjects who are responsible for this despicable crime, and we will do everything we can to bring them to justice."

Doctors have revealed the extent of the injuries suffered by those caught in the blasts, including details of a nine-year-old girl who had lost her leg and a 10-year-old boy who suffered deep shrapnel wounds.

Several people have had to have limbs amputated and others are at risk of losing legs following the blasts that ripped through crowds during the city's marathon.

Doctors at Massachusetts General Hospital, one of several hospitals where victims were taken, told reporters eight people were in a "severe" condition.

Alisdair Conn, chief of emergency services, said: "This is something I've never seen in my 25 years here ... this amount of carnage in the civilian population. This is what we expect from war."

A total of 17 people remain in a critical condition.

The first victim killed in the blasts in the heart of the city to be named was eight-year-old Martin Richard, who lived in Boston.

Video:Witness Films Second Boston Blast

Martin had been waiting with his brother, sister and mother at the finish line for his father, who was running the race.

The bombs went off within seconds of each other and about 100m (330ft) apart on the same street, blowing out windows and sending smoke and debris into the air.

The explosions happened four hours into the race and about two hours after the men's winner had crossed the line, as amateur runners were reaching the finish.

More than 17,000 competitors had completed the race by the time the blasts struck.

Video:How Boston Drama Unfolded

Police commissioner Ed Davis said the area around the blast areas is the most complex crime scene in history of the department.

Authorities are now looking for amateur video and photographs that can give clues to who set off the bombs.

Security has been stepped up in Washington and New York, and Boston itself is a city on high alert, with armoured vehicles seen on the streets on Tuesday and random checks of backpacks on public transport.

Earlier, a plane was grounded at the city's Logan Airport over security concerns.

Video:Boston Police Search Apartment

There have been no immediate claims of responsibility for the attack, the most serious in the US since the 9/11 World Trade Centre atrocity. Al Qaeda-linked groups and militant white extremists have attacked targets in America in the past.

The Pakistani Taliban, who have previously threatened attacks in the US, have denied any involvement.

Police are said to have questioned a 20-year-old Saudi Arabian man who is being treated for injuries at a hospital in Boston. Officers have searched his apartment in Revere, according to his flatmate.

More than 25,000 people were registered as taking part in the race, 374 of whom were British. There were also 108 Irish athletes.

The British Foreign Office has said it is not aware of any British nationals who have been injured but that it will continue to monitor the situation.

Police in London have reviewed security plans for Sunday's London Marathon. London's Met Police Commissioner Bernard Hogan-Howe said authorities were sharing information with colleagues in America.

Speaking at a news conference today, he said there would be greater security checks and more police officers on the streets for the event.

The London race's chief executive, Nick Bitel, said it was "a very sad day for athletics and for our friends in marathon running".

Video:Brits Runners Describe Boston Blast

There will be a 30-second silence ahead of the start to honour the victims of the Boston bombings. Runners will also be encouraged to wear black ribbons.